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ATTENBOROUGH: The natural world
is full of extraordinary animals

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with amazing life histories.

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Yet, certain stories
are more intriguing than most.

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The mysteries
of a butterfly's life-cycle

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or the strange biology
of the emperor penguin.

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some of these creatures were surrounded
by myths and misunderstandings

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for a very long time.

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And some have only recently
revealed their secrets.

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These are the animals
that stand out from the crowd.

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The curiosities 1 find
most fascinating of all.

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spiders spin intricate webs
using their own silk

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and birds weave nests
from strips of leaves.

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1n this programme,

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1 investigate the skill
of these spinners and weavers

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and the way they use such materials

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to produce
such truly complex structures.

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(CHIRPING)

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Birds build a variety of nests,

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each with a design that
is characteristic of their species.

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The simplest nests are
just sticks wedged into position,

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but some are more complicated.

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The long-tailed tit builds
a delicate nest

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from plant material and spider silk

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and weaver birds do
literally weave with leaves.

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But are such skills learned
or instinctive?

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In 1 905, Eugene Marais,
a South African writer and scientist,

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was intrigued by the complexity
of weaver bird nests.

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He wanted to understand more
about their nest-building skills

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and performed a rigorous
but simple experiment

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to see if they learnt how to make nests

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or if they built them using
what he called ''cultural instinct.''

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He took eggs from a pair
of wild weaver birds

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and put them
into a canary's nest to hatch.

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Then he encouraged
the next three generations

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of weaver birds to breed,
but gave them no nest material

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and hatched their eggs
once again on the canary's.

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When nesting time came for the
fourth generation of weaver birds,

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he gave them natural nest materials
and, without hesitation,

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they vigorously set about
constructing perfect wild nests.

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so nest-building is largely
under genetic control,

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but it is influenced by experience
and the environment.

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Nests of the same kind of weaver bird
are not always exactly the same,

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and the birds, of necessity,

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must have some flexibility
in how they build.

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Nests that hang are
particularly difficult to make,

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as the birds have
to work against gravity,

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with no support from below.

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Weaver birds solve part of this problem
with a skill none others have.

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They're the only birds
that can tie knots.

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These knots vary and are worked on
until the weaver succeeds

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in attaching several strands of grass
to a suitable branch or stone.

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These first fastenings are crucial,

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as the whole of the completed nest
will hang from them.

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Once the birds have
secured the foundation,

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they can start to weave.

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Weaving is just one way
of binding leaves together.

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There are others.

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These are tailorbird nests.

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They consist of folded leaves
stuffed with soft material

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and stitched together using spider silk.

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The tailorbird pierces the leaves
with its sharp beak

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and then binds them together
by pulling silk through the holes.

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The complete operation involves
a number of different skills.

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Making the holes is like riveting.

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Two leaves are placed together

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and then pierced to create
matching holes above and below.

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Then the edges are sewn up.

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The upper surface of the leaf

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is kept to the outside to help
the nest look unobtrusive.

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The result is a secure pocket,

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which is then stuffed
with a soft lining.

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The materials the birds choose
to sew up their nest can vary.

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At the turn of the century,

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there was a report
in The Common Birds of Bombay

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of weaver birds watching carpet-makers
and tailors

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as they worked on verandas.

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When the coast was clear,

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the birds flew down
and stole tiny pieces of thread

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with which to sew up their nests.

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(SQUAWKING)

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Birds search with a clear idea

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of what would be suitable nest material.

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Many use sticks and twigs.

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(CHIRPING)

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They will, however,
occasionally use other material

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that does the same job

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and their choices are
sometimes surprising.

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This nest was found
in an aircraft hangar in the 1 950s,

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and is made entirely of twisted wire.

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When it was discovered,
it contained two blackbird eggs.

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1t's an unusual nest for a blackbird,

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but similar nests have been found
belonging to crows and pigeons.

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Weaver birds work with natural material

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and, like the tailorbird,

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they have to solve the problem
of joining leaves together.

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After making a knot to secure
the basic framework,

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they begin their weaving.

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They construct the main egg chamber

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and then add a small entrance

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around the first securely-knotted ring
of leaves.

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The male, as he works,
is under intense scrutiny.

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Females are looking for mates,
and males that build firm,

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well-positioned nests
are favoured as fathers.

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When he finishes,

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a male advertises his handiwork
by fluttering.

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But he may be forced
to build several nests

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before a female finally chooses him
as a partner.

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Weaver birds' nests are
very conspicuous.

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Other birds, however,
go to some trouble to conceal them.

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We may not have tailorbirds
or weaver birds in Britain,

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but we do have long-tailed tits,

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delicate little birds that make
intricate and finely-constructed nests.

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With tiny repetitive movements,
they use loops of spider silk

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to felt together a mixture
of wool and moss.

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Both male and female work
on the construction.

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As their nest takes shape,

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they decorate the outside with
several thousand tiny flakes of lichen.

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The nest is then lined
with hundreds of feathers

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and provides a delicate
but strong structure

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to house the growing chicks.

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(CHIRPING)

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And it's a nest
that's particularly hard to find,

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because of its covering of lichen.

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For years, it was believed

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that this acted as a sort of camouflage

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to help hide the nest,

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but the recent discovery
of long-tailed tit nests

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covered with small flakes of paper
and polystyrene

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have helped explain more clearly
the reason for this decoration.

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Rather than helping to blend
the nest with its background,

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these small flakes reflect
light from it,

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making it almost invisible,

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and it seems paper
and polystyrene do the job

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just as well as lichen.

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The largest and perhaps
the most long-lasting nest of all

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is made by the social weaver bird.

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(BIRDS CHIRPING)

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They live in the dry areas
of southern Africa

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and work together to build
what looks like a great haystack

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up in a tree.

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New nest chambers are continually added.

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As many as a hundred pairs of birds
may live together

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under the one roof, as you might say.

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The chambers provide shade
during the day

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and keep out the chill at night.

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And the whole construction is so robust

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that it may provide mass housing

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for generation after generation
of birds.

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Recently, the biggest nest ever recorded

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was discovered attached
to telegraph poles

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in the Kalahari Desert.

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1t's more than seven metres across
and three metres high.

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so weaver birds make their nests
in many different ways

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and it was once thought that
they worked entirely by instinct,

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but this is not so.

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They are amongst
the most expert nest-builders

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in the animal kingdom.

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And this array of nests
shows the complex

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and the elaborate designs
that they can produce.

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Recent studies suggest that weaver birds
may be using mental skills

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that are not dissimilar to those
required to make simple tools.

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For weaver birds,

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a well-built nest is a ticket
to successful breeding.

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Who would imagine that such complexity

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could be produced
using just a foot and a beak?

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Weaver birds make their elaborate nests
from simple materials

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they find around them.

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Another of nature's
extraordinary builders

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are the spiders.

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They make their complex webs

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from an incredible substance
they produce themselves, silk.

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Spider silk is unique.

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It's very thin, very strong,

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and has many exciting potential uses.

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spiders spin it with ease,

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but scientists have been trying
to copy it for many years.

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To do that, we need to understand
two of the spider's secrets.

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The exact structure and nature
of their silk

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and the way they transform it
from a fluid into a thread.

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spider silk is
a truly remarkable material.

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1t can withstand impact
and it can be strong, stretchy,

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and sticky all at the same time.

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spiders produce it from special glands
inside their bodies

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and extrude it from tiny nipples
called spinnerets

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at the back end of their abdomens.

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And what is more,

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they can produce
up to seven different kinds,

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each with its own purpose.

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For centuries,
it was the only silk known to man.

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The ancient Greeks used cobwebs
to stop bleeding

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and Australian Aborigines used it
to catch small fish.

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Then, in the Far East,

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a different and mysterious
new kind of silk

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started to appear
and in much larger quantities.

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According to Chinese legend,

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the first person to weave silk
into a fabric

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was the Empress Leizu
back in the 2 7th century BC.

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She was having tea in her garden
under a mulberry tree,

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when a cocoon fell from the branch above

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and dropped into her cup

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and started to unravel.

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Whether that's true or not,

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the Empress Leizu is now honoured
as the goddess of silk,

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and silk moth farming dates

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back to the beginning
of Chinese civilisation.

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The silk was traded
right across the Near East

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and into the Roman Empire.

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The Chinese traders were
sworn to secrecy

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about how this marvellous material
was made.

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But in the year of 532,

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the Roman emperor justinian
managed to find out

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that it came not, as some suspected,
from a spider's web,

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but from the cocoon of a moth.

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silk moth caterpillars produce
large quantities of silk

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and they make it
in a very different way to spiders.

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The caterpillars feed voraciously
on mulberry leaves

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and then, when they're full-grown
and ready to transform into a moth,

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they spin silken cocoons
in which they will pupate.

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Unlike spiders,
which have specialised spinning organs,

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silk moth caterpillars produce silk
from their salivary glands.

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Each cocoon is made
from a single unbroken filament

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that can be over 500 metres long.

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This silk is plentiful
and easy to spin commercially,

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but it isn't as tough as spider silk.

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And spider silk also has
more exciting potential uses.

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An orb web like this is constructed
over a ''Y''-shaped scaffold

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of silk threads,
which are extremely strong.

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Unlike silkworms, the female spiders,

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which spin the webs,
are very territorial and aggressive.

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So farming it and collecting
spider silk is very difficult,

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but it has been done.

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1n 1 762, a spanish missionary
called Termeyer

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made a machine that held a single spider

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from which he pulled a silken thread.

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1n London, Daniel Rolt,

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a factory worker, attached spiders
to a small steam machine

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and succeeded in reeling out
1 8 metres of silk a minute.

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That led to machines

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which are able to milk
several spiders at a time.

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Experiments then stopped until 2004,

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when two textile artists
in Madagascar built a machine

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based on these early designs

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with which they made
something very special indeed.

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The golden colour of this stunningly
beautiful spider silk shawl

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is completely natural.

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The silk from which it was made

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was produced by 1 ,06 3,000 spiders,

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like this one, over four years.

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Local people collected
3,000 spiders a day

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and trained handlers extracted silk
from groups of 24 at a time.

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After being milked,

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the spiders were released
back into the wild.

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The individual silk strands
were then twisted into a thread,

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which was woven into this intricately
patterned fabric on looms.

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00:16:35,207 --> 00:16:38,995
Now, this kind of silk fabric production
couldn't work commercially.

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Apart from being hard work
to make in quantity,

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spider silk isn't really
a very suitable thread for fabric.

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As a cloth, it reacts badly
to moisture and heat,

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but in its natural state
as a single thread,

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it has physical qualities that
could be exploited medically.

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These special characteristics
are the consequence

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of the molecular structure
of spider silk.

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1t consists
of two large protein molecules.

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One is stretchy and spaghetti-like

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and the other has
a harder, crystalline structure.

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Combined, these two proteins give silk

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unique qualities of strength
and flexibility.

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spiders store these proteins
as a gel-like liquid in their bodies.

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And when they need to make silk,

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they extrude it through the spinnerets,

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combining the molecules
in a special way.

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1f we hold down a spider
without harming it,

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we can see this process in more detail.

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Normally,
the spider would attach the end

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of the silk filament to an object

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and then move away,

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00:17:46,927 --> 00:17:49,805
so that the filament is
pulled from her spinnerets.

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We can produce the same reaction

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by gently pulling the end
of the filament itself.

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Internally, the silk liquid is
passing down a long duct

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in which stretchy elements
within the protein molecules

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are aligned with harder crystalline ones

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to create an extremely strong
and tough thread.

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scanning electron microscopes

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reveal how the liquid emerges
from the spinnerets.

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00:18:18,207 --> 00:18:21,722
1ncredibly,
spiders can convert liquid proteins

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into a hardened thread
at room temperature

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with very little energy.

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00:18:26,767 --> 00:18:29,600
1f we could understand
and copy this process,

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it would be
a major scientific breakthrough.

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00:18:34,647 --> 00:18:36,842
scientists have, in fact,
spent many years

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trying to replicate
the spider's liquid silk

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and the way it's spun.

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00:18:41,287 --> 00:18:44,404
Recently,
the genes of spider silk proteins

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were cloned and put into goats

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to try and produce silk in their milk.

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00:18:50,367 --> 00:18:52,961
1t worked and, when the goats had kids,

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silk proteins were extracted
from the mother's milk.

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00:18:58,887 --> 00:19:01,879
But none of these processes
have yet produced silk

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that is as tough as natural spider silk.

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This machine is called a tensile tester

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00:19:09,887 --> 00:19:13,516
and it shows how strong
and stretchy spider silk can be.

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This drag line silk
is being pulled apart

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00:19:17,967 --> 00:19:20,606
and a graph shows
the force the fibre is taking

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00:19:20,687 --> 00:19:22,837
and at what point it breaks.

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A steel thread of similar diameter
would've broken by now.

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00:19:33,127 --> 00:19:34,845
There, it's broken.

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00:19:35,207 --> 00:19:39,962
Spider silk is the toughest
natural material known to man.

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00:19:43,247 --> 00:19:47,160
A single thread of web silk
less than a millimetre thick

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00:19:47,247 --> 00:19:50,205
can absorb the impact
of fast-moving prey

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and bring it to a halt without breaking.

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00:19:53,567 --> 00:19:56,479
Complete webs can stretch enormously

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and then return to their original shape
with a minimum of damage.

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1ncredibly, spiders can make
this complex material

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00:20:07,047 --> 00:20:09,925
from just fresh air, flies and water.

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00:20:11,927 --> 00:20:14,646
The best we can do
in making a material like it

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00:20:14,727 --> 00:20:18,402
requires oil, chemicals,
and a great deal of energy.

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00:20:21,287 --> 00:20:24,723
Although we now better understand
the structure of spider silk

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00:20:24,807 --> 00:20:26,718
and the natural spinning process,

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00:20:26,807 --> 00:20:29,401
we still can't perform
the spider's magic

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00:20:29,487 --> 00:20:32,081
and copy this extraordinary substance.

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00:20:33,167 --> 00:20:37,126
But using small amounts
of natural spider silk in clever ways

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00:20:37,207 --> 00:20:40,005
has, nonetheless,
a very exciting future.

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00:20:42,007 --> 00:20:46,398
A sumptuous golden cloth
is just one possible product.

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00:20:48,087 --> 00:20:51,796
This is the dream
that has become a reality

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00:20:51,887 --> 00:20:54,845
and shows just how lovely
spider silk can be.

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00:20:56,607 --> 00:21:00,395
But it also has the potential
to make other dreams come true.

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00:21:01,127 --> 00:21:03,800
1t's a biodegradable material

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00:21:03,887 --> 00:21:07,004
that we are now using
to make artificial joints

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00:21:07,127 --> 00:21:10,483
and it may even help
repair damaged spinal tissue.

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00:21:12,487 --> 00:21:16,844
This curiosity of nature
could eventually save lives.